1. Field
The present invention relates generally to magnetic memory systems and, more particularly, to a method and system for providing an element that employs a spin-transfer effect in switching and that can be used in a magnetic memory, such as a magnetic random access memory (“MRAM”).
2. Description of the Related Art
Magnetic memories are often used for storing data. One type of memory element currently of interest utilizes magneto-resistance of a magnetic element for storing and reading data. FIGS. 1 and 2 depict conventional magnetic elements 100 and 200.
The conventional magnetic element 100, shown in FIG. 1, is a spin valve (SV) 100 and includes a conventional anti-ferromagnetic (AFM) layer 110, a conventional pinned layer 108, a conventional spacer layer 106, which is typically a conductor, and a conventional free layer 104. The conventional pinned layer 108 and the conventional free layer 104 are ferromagnetic. The conventional spacer layer 106 is nonmagnetic. The AFM layer 110 is used to fix, or pin, the magnetization of the pinned layer 108 in a particular direction. The magnetization of the free layer 104 is free to rotate, typically in response to an external field. Contacts, such as a bottom contact 112 and a top lead 102, can be coupled to the magnetic element 100 to provide electrical contact to the magnetic element 100.
The conventional magnetic element 200, shown in FIG. 2, is a magnetic tunneling junction (MTJ). Portions of the MTJ 200 are analogous to the conventional spin valve 100. Thus, the conventional magnetic element 200 includes an anti-ferromagnetic layer 210, a conventional pinned layer 208, an insulating barrier layer 206, and a free layer 204. The conventional barrier layer 206 is thin enough for electrons to tunnel through in a conventional MTJ 200. Contacts, such as a bottom contact 212 and a top lead 202, can be coupled to the magnetic element 200 to provide electrical contact to the magnetic element 200.
Depending upon the orientations of the magnetizations of the free layer 104 or 204 and the pinned layer 108 or 208, respectively, the resistance of the conventional magnetic element 100 or 200, respectively, changes. When the magnetizations of the free layer 104 and pinned layer 108 are parallel, the resistance of the conventional spin valve 100 is low. When the magnetizations of the free layer 104 and the pinned layer 108 are anti-parallel, the resistance of the conventional spin valve 100 is high. Similarly, when the magnetizations of the free layer 204 and pinned layer 208 are parallel, the resistance of the conventional MTJ 200 is low. When the magnetizations of the free layer 204 and pinned layer 208 are anti-parallel, the resistance of the conventional MTJ 200 is high.
In order to sense the resistance of the conventional magnetic element 100, 200, current is driven through the conventional magnetic element 100, 200. Current can be driven through the conventional magnetic element 100 in one of two configurations, current in plane (“CIP”) and current perpendicular to the plane (“CPP”). However, for the conventional magnetic tunneling junction 200, current is driven in the CPP configuration. In the CIP configuration, current is driven parallel to the layers of the conventional spin valve 100. Thus, in the CIP configuration, current is driven from left to right or right to left as seen in FIG. 1. In the CPP configuration, current is driven perpendicular to the layers of conventional magnetic element 100, 200. Thus, in the CPP configuration, current is driven up or down as seen in FIG. 1 or FIG. 2. The CPP configuration is used in MRAM having a conventional magnetic tunneling junction 200 in a memory cell.
Recently, a spin transfer effect has been proposed as a switching mechanism for magnetic memory elements. See J. C. Slonczewski, “Current-driven Excitation of Magnetic Multilayers,” Journal of Magnetism and Magnetic Materials, vol. 159, p. L1–L5 (1996). In original spin transfer systems, a Co/Cu/Co pseudo-spin valve with current perpendicular to the plane (CPP), similar to that shown in FIG. 1 (but without the AFM layer 210), was used. See L. Berger, “Emission of Spin Waves by a Magnetic Multilayer Traversed by a Current,” Phys. Rev. B, Vol. 54, p. 9353 (1996), and F. J. Albert, J. A. Katine and R. A. Buhman, “Spin-polarized Current Switching of a Co Thin Film Nanomagnet,” Appl. Phys. Left., vol. 77, No. 23, p. 3809–3811 (2000).
However, using such a spin transfer system presents two primary challenges. First, the current required to induce the switching is high, e.g., on the order of 1 mA or greater. Second, the output signal is small, such that both the total resistance and the change in resistance in SV-based spin transfer elements are small, e.g., normally less than 2 Ohms and 5%, respectively.
One proposed method of increasing the output signal is to use a magnetic tunnel junction (MTJ) for the spin transfer device, similar to that shown in FIG. 2. See J. C. Slonczewski, “Current-driven Excitation of Magnetic Multilayers,” Journal of Magnetism and Magnetic Materials, vol. 159, p. L1–L5 (1996). The magnetic tunnel junction can exhibit large resistance and large signal, e.g., >1000 Ohms and >40% dR/R, respectively. However, this approach still cannot decrease the high operating current sufficiently.
It should be apparent from the discussion above that there is a need for a device and method for providing a magnetic memory element that consumes low power such that it can be used in a high density memory array. Further, there is a need to provide a device and method for protecting sensitive layers of the MTJ from the relatively high write current. The present invention satisfies this need.